The present disclosure relates generally to polyurethane foams that are resistant to discoloration. In particular, the present disclosure relates to flexible polyurethane foams with improved resistance to discoloration from oxidation, such as oxidation due to exposure to oxides of nitrogen in burnt gas fumes.
Polyurethane foams and methods for manufacturing such foams are well known. Fundamentally, a polyurethane foam is manufactured from polyisocyanate, surfactant, and polyol. In addition, manufacturing usually requires a catalyst, typically comprising a tertiary amine component and/or an organo-tin component, and a blowing agent. Additional components such as, but not limited to, fillers, pigments/dyes, flame retardants, biocides, and/or antistatic agents are also frequently added depending on the desired end-use of an article that incorporates the foam. Different combinations of polyol, polyisocyanate, and surfactant tend to give different properties to the resultant polyurethane foam. Polyurethane foams may vary from being flexible to semi-rigid to rigid, having open cell structures to relatively closed cell structures, porous to non-porous, and may have a wide range of other physical properties, depending on the requirements of the end-user.
Polyurethane foam generally exhibits a high propensity to discolor during manufacture and storage due to numerous factors, including oxidation discoloration from exposure to oxides of nitrogen. Oxides of nitrogen that readily attack polyurethane foam to cause discoloration are often created through gas engine combustion. Such engines are often prevalent in the manufacturing environment, as well as in the general environment.
The manufacture of an aesthetically acceptable polyurethane foam that also prevents discoloration from oxides of nitrogen and other sources (e.g. UV light exposure, phenolic yellowing) has proven difficult, no matter what methods or compositions have been employed. For example, the use of aliphatic isocyanates has been known to allow the creation of polyurethane foam with significantly reduced or no discoloration upon exposure to ultraviolet light. U.S. Pat. No. 4,067,832 to DesMarais provides an example of such foams, the disclosure of which is hereby incorporated by reference in its entirety. However, aliphatic isocyanate-based foams are difficult to process and reproduce with consistent physical and aesthetic properties.
Due to the lower reactivity of aliphatic isocyanate compared to standard aromatic isocyanates, prior attempts to use aliphatic isocyanate have included the use of heavy metal catalysts (such as lead) to provide the catalysis necessary to make these foams. See, for example, U.S. Pat. No. 4,150,206 to Jourquin et al. and U.S. Pat. No. 4,607,062 to Megna, the disclosures of which are hereby incorporated by reference in their entireties. Such efforts have not been successful for a variety of reasons, including the safety concerns of end-products potentially containing these heavy metal compounds. In addition, aliphatic isocyanate-based foams are problematic due to their significantly increased costs compared with foams made with standard isocyanates, such as aromatic isocyanates.
One aliphatic isocyanate for use in polyurethane foams is supplied by Recticel of Evere, Belgium, under the trade name Bulfast®. This isocyanate is marketed for applications that require articles to have a discoloration resistance to ultraviolet light. However, these foams generally still discolor unacceptably when exposed to other sources of discoloration, such as oxides of nitrogen. Furthermore, these and other aliphatic isocyanate-based foams are known to exhibit poor or commercially unacceptable hydrolytic stability, resulting in the loss of any physical form when the foams are exposed to high temperatures and humidity for prolonged periods of time.
The polyurethane industry has also examined the use of various stabilizers and/or antioxidants to overcome persistent discoloration problems. However, stabilizers and/or antioxidants are not a cure-all to the problems associated with polyurethane foam discoloration. Stabilizers and/or antioxidants often pose problems in making foam easy to process. For example, the incorrect type or incorrect quantity of additives will prevent polyurethane foam from possessing and displaying industry-acceptable physical and aesthetic properties.
Therefore, there exists a need in the art for a combination of additives to be used in a standard polyurethane foam system (i.e., made from aromatic isocyanates) to produce foam resistant to discoloration from oxides of nitrogen.